Apparatus for generating and utilizing frequency-swept phonons

ABSTRACT

There are disclosed a variety of apparatuses for varying the order parameter of a superconductor in order to generate frequency-swept phonons, these phonons being used to investigate the properties of a sample of material capable of coupling the phonons from the superconductor. In respective embodiments the order parameter, and hence the superconductive energy gap, of the superconductor is varied by a pulsed laser beam, a direct-current pulse, and R-F field pulse in a strip line and a magnetic field pulse from an electromagnet. Resonant acoustic absorptions of the sample are observed as the phonon frequency varies.

United States Patent I 1 UN Inventors Cecil A. Nanney Murray Hill; PingK. Tien, Chatham Township; Morris County, both of, NJ.

App]. No. 778,286

Filed Nov. 22, I968 Patented June 8, I971 Assignee Bell TelephoneLaboratories, Incorporated Murray Hill, NJ.

APPARATUS FOR GENERATING AND UTILIZING FREQUENCY-SWEPT PHONONS 8 Claims,5 Drawing Figs.

U.S. Cl 73/672, 330/155, 333/30 Int. Cl G0ln 29/00 Field of Search73/679,

MODULATOR 23 FLASHLAMP ELECTRO-OPTIC [5 6] References Cited OTHERREFERENCES Einspruch, N. G. Ultrasonic Studies of Solids. From IEEESpectrum March I966. pages I I6 I24. Copy in 330/5.(5)

Primary Examiner-Jerry W. Myracle AtrorneysR. J. Guenther and Arthur J.Torsiglieri SUPERCOND. I6

VARIABLE HEIGHT VARIABLE WIDTH PULSE "SOURCE PUMPING SOURCE DETECTORDIODE l3 WINDOW DEWAR- I7 SAMPLE I2 DISPLAY DEVICE PATENIEBJUN aIIm3583212 SHEET 1 BF 2 I SDPERGDND FI GJv 2 I ELECTRO-OPTIC l6 FILM 23FLASHLAMP MODULAT2O5R Q g I 28 DETECTOR (26 24/?IINDOW DIODE I3 PUMPINGVARIABLE SOURCE HEIGHT w VARIABLE v 22 SAM PLE I DISPLAY 29 SOURCEDEV'CE m SAMPLE FIG. 2

DIRECT- cuRRENT 351%? PULSE k 7 M GEN. DEWAR IS suPERcoND.

DETECTOR DIDDE II I3 FIG. 3

DIELECTRIC DISPLAY I, III," I 45 DEVICE I LL'PH'ELfl PU LSED l l 21 L RF15.1"; T FIELD SOURCE v 1? f\ TERMINATION '\1 B V/ /W l Y/ I fi/ FIELDMAGNET. 44 SAMPLE DETECTOR 52 DIDDE l3 I7 SUPERCONDL C. A NA/VNEV FILMINVENTORS 0 TIME (IN REGION OF By WEN MAxMAGNETIc FIELD CUWUM ATTORNEVPATENTEUJUH sum 3583212 SHEETZUFZ DISPLAY DEWAR SUPERCOND. DETECTOR 5 2FILM DIODE RESONANT ABSORPTION BY SAMPLE DETECTED PHONON SIGNAL TIME ORFREQUENCY APPARATUS FOR GENERATING AND UTILIZING FREQUENCY-SWEPT PHONONSBACKGROUND OF THE INVENTION This invention relates to an apparatus forgenerating and utilizing frequency-swept phonons. Phonons are packets ofacoustic energy. The concept of a packet of acoustic energy, ascontrasted to the concept of acoustic waves, is particularly useful forthe higher frequencies of acoustic energy. Nevertheless, the termsphonons and acoustic waves can be used nearly interchangeably.

In the copending patent application of A. H. Dayem at al., Ser. No.586,247, filed Oct. 12, I966, now U.S. Pat. No. 3,405,374 and assignedto the assignee hereof. there is disclosed the generation of essentiallymonochromatic streams of phonons by tunneling in superconductive diodes.In their experiments, the tunneling creates so-called quasi-particles inthe second layer of the superconductive diode. The quasi-particles firstrelax to the top of the superconducting energy gap and then emit phononsof frequency corresponding to the energy ofthe gap by recombination.Since phonons ofthe very high frequencies thus obtained may be usefulfor investigating the properties of materials, alternative arrangementsfor generating the quasi-particles in superconductors under conditionsappropriate for obtaining the desired phonons is of continuing interest.

In particular, in the laboratory the experimentalist often needs afrequency-swept phonon generator whose frequency varies in a continuousmanner. This need can be appreciated if one considers that many useshave been found in the past for the chirp microwave generators found insubstantial variety in microwave technology. Thus, in contrast to theapparatus of the above-cited copending patent application. we desire aphonon generator whose frequency is a varying function of time, butwhich ata given instant emits phonons ofa constant frequency.

SUMMARY OF THE INVENTION According to our invention, this objective isachieved in a single superconducting thin film by applying energy to thefilm to excite quasi-particles while simultaneously varying thesuperconducting energy gapof the film. This variation of thesuperconducting energy gap'can be described as a variation of the orderparameter of the superconductor. This variation is achieved byappropriately pulsing a direct current in the superconductor or amagnetic field in the superconductor or by heating or cooling thesuperconductor, preferably without driving it into its so-called normal,or nonsuperconducting, state. Heating of the superconductive film canbeachieved illustratively by a pulsed laser beam. The pulse ofenergycauses the phonons generated to vary infrequency continuously with time,from a first frequency to a second higher frequency during each pulse ofexcitation.

BRIEF DESCRIPTION OF THE DRAWINGS Further details and advantages ofourinvention will become apparent from the following detaileddescription, taken together with the drawing in which:

FIG. I is a partially pictorial and partially block diagrammaticillustration ofa first embodiment of our invention;

FIG. 2 is a partially pictorial and partially block diagrammaticillustration ofa second embodiment employing a directcurrent pulse;

FIG. 3 is a partially pictorial andpartially block diagrammaticillustration of a third embodiment of the invention employing a pulsedR-F field;

FIG. 4 is a partiallyschematic andpartially blockvdiagrammaticillustration of another embodiment of theinvention employing a pulseddirect-current magnetic field;-and

FIG. 5 shows a curve helpful in explainingthe use.of theinvention ininvestigating the properties of materials.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS The phonons generated in thefollowing specific embodiments are of very high frequency lying in therange from l0 to 10 Hz., where the generation of phonons by conventionaltechniques is difficult. The cycle of generation may be repeated atfrequencies as high as about 10" Hz. Several milliwatts of power may begenerated in this manner.

In the embodiment of FIG. 1, a superconducting film II in which thephonons are to be generated is disposed on a sample 12 of material, theproperties of which are to be investigated. On an opposed surface ofsample 12 a superconductive diode I3 is disposed to act as a detector.For this purpose, the detector diode I3 is connected across a biassource 14 and a suitable display device 15 such as a cathode-ray tube.The film ll, sample 12 and detector diode 13 are immersed in liquidhelium 16 contained in the cryogenic Dewar 17. At one wall of Dewar 17there is provided an optically transparent window 18 in order to admit acoherent light pulse from a laser 19.

The laser 19 comprises the solid state active medium 20 with'Brewsterangle end surfaces, the pumping means includ ing the Flash lamp 2] andpumping source 22 and the optical resonator comprising reflectors 23 and24 disposed about the active medium 20 and aligned along a common axis.Within the resonator thus formed, an electro-optic modulator 25 isdisposed in optical alignment with the active medium 20. It includes theelectro-optic medium 26 and the transparent electrodes 27 and 28,through which a modulating voltage is applied from a source 29.Specifically, the modulating voltage is of a magnitude which can rotatethe polarization of the coherent radiation by to extinguish the laseroscillation. It is a pulse which should have a width shorter than theperiod of repetition rate desired of the phonon generator. Thus, theelectro-optic modulator 25 acts as a shutter for the coherent light.

Specifically, the film I] may be a lead superconducting thin film ofthickness of 5,000 A., which is less than half of the socalled Londonpenetration depth. The liquid helium provides a temperature ofthe filmII of l.5 Kelvin. The superconducting detector diode I3 is a sandwich ofsuperconducting lead, an insulator such as aluminum oxide and, finally,another layer of superconducting lead. The voltage of source 14 is about2 millivolts, which is slightly less than one-half of thesuperconducting energy gap of the lead layers of diode 13. In otherwords, it is much less than the so-called Fermi energy. The sample 12may illustratively be a sapphire crystal.

The presence or absence of resonant absorption is to be determined byobservation of the display on the cathode-ray display device 15.

The laser 19 may illustratively be a typical ruby laser in which theelectro-optic modulator 25 is a potassium dihydrogen phosphate (KDP)electro-optic modulator.

In operation, letus assume that a repetition rate of 10 cycles persecond of frequency-swept phonons in the range from 10' to l0 cycles persecond is desired. For an output pulse power of I00 milliwatts asapplied to film 11, the width of the pulse can be no greater than about0.5 microseconds in order not to drive film ll into its normal state.Thus, the electrooptic modulator 25 must block laser oscillations for.the remaining 9.5 microseconds out of the 10 microseconds of each cycleof the repetitive operation. All of the foregoing parameters are-merelyillustrative of very broad ranges of parameters that would satisfy theprinciples set out above.

Asa result of heating by the laser pulse, the superconducting energy gapof film II will be reduced to approximately 0.2 millielectron volts atthe end of the laser pulse. Quasi-particles have been excitedin film 11by the laser pulse and will now release and emit phonons byrecombination. At the first instantof time during the recombinationprocess, the phonon frequency will be illustratively 2X10" Hz. Ouranalysis, set out hereinafter, shows that the phonon frequency willincrease nearlylinearly with time as the superconductive film 11experiences an increase in its order parameter and a correspondingincrease in its superconducting energy gap. microseconds later thephonon frequency is now 2X10" Hz. At this time a new laser pulse isapplied to the superconducting film II, and the superconducting energygap is again reduced. Each laser pulse starts a new cycle ofoperation.

If it is desired to change the frequency range covered by thefrequency-swept phonons, the starting frequency of the phonon generationcan be changed by changing the product of pulse height and pulse widthfrom laser 11; and the final frequency of the sweep can be changed bychanging the repetition rate of the operation by changing the durationof the time period in which the coherent laser is blocked byelectro-optic modulator 25.

As the following calculations will show, the spectrum of the phonongenerated can be made uniform over a significantly large range offrequencies. This feature is an important advantage for a variablephonon source.

THE SPECTRUM OF THE PHONON EMISSION We shall consider first the case inwhich the phonons are generated by cooling a superconductor fromtemperature T to a lower temperature T. In order to calculate this, wemust first compute the number of quasi-particles in the superconductorsat different temperatures. According to the theory set out in thearticle by J. Bardeen et al., Physical Review, Vol. 108, i175 (1957),the density of state of a quasiparticle of energy E is: E

where 26,,( T) is the energy gap at the temperature T and E=(c +c(T))l/2. The quantity N(0) is the density of states per unit energyinterval at the Fermi surface. The factor two comes from two spins foreach energy state. Now the quasiparticles are distributed themselvesamong the energy states according to the Fermi-Dirac distribution,

We have, therefore, the number of quasi-particles in the superconductorat the temperature, T, as

v m E 1 l..u- 0 W)? 1 '+e Let X=E/KT and KT e,,(0) KT T 6 (0) T (4) Herewe have used the relation 2e;, (0 )=3.5 KT}, where 2e,,(0) is the energygap at 0 K. The integral I is then reduced to Since b, and e,,(T7/e,,(0)are functions of T/T only, we can show that I/4N(0)KT is a nearly lineardecreasing function of e,,(T)/e,,(0) for e,,(T)/e,(0) greater than about0.5. This function should be universal to all the superconductors. It isactually somewhat surprising to discover that between the function isalmost linear. One might expect the function to be exponential;specifically, the worker in the art would expect the number ofquasi-particles should decrease exponentially as e,( 77/6,,(0)approaches to unity (or as KT approaches to zero). This is not sobecause ofthe nonlinear dependence of e,,(T7/e,,(0)'with TlT As will beseen later, this almost linear relationship between the number of thequasi-particles and e,(T7/e,,(0) has made this chirp phonon generatorvery attractive.'

As we cool the superconductor, the energy gap increases as thetemperature decreases; and so the number of quasi-particles excited inthe superconductor decreases. The lost quasiparticles are removed byrecombination and emit phonons whose frequency corresponds to the energygap of the superconductor. such as,

h HFZQK T). (6)

where w is the phonon frequency. Since the energy gap varies with thetemperature in the cooling process, we have thus a variable frequencyphonon generator. Moreover, we see the phonon frequency is proportionalto c,,( T)/e,,(0). Considering a specific example, if we cool thesuperconductor from T/T =0.5 to 0.9, we find from the figures that thenumber of the quasi-particles decreases from 0.45X4 N(0)KT "to 0.05X4

xN(0),.. Each pair of the quasiparticles lost generates one phonon; wehave thus for the number of the phonons generated,

/(0.450.05) 4byPn(0) -KT,. (7) For lead N(0)= I0lerg-cm. and T,=7.22 K,we have then /& (O.450.05) 4 0.92 i0=" 7.2 l33x10 or 7.3 l0 phonons/cm".Consider a thin film 9% cm. X 5% cm. X 1,000 A. and let us repeat thecooling and heating cycle l0 times per second, we generate I.82X l0phonons per second. Since the curve in FIG. la is almost linear in therange considered, the generated phonons are uniformly distributedbetween the frequency hw=l.05 6 (0) to hw=l.92 6 (0). As another examplefor an aluminum film under the same conditions, N(0)=l.2lXl0/erg-cm. andT =l.l4 K, we have 3.8X l 0" phonons per second.

OTHER EMBODIMENTS In FIG. 2 the order parameter of the superconductingfilm 11 is varied by a direct-current pulse from a suitable generator31, which is connected across the film 11. The remaining components ofthe apparatus are the same as in FIG. 1 except that window 18 is nolonger needed. As in the embodiment of FIG. 1, reduction of the orderparameter and the superconducting energy gap occur during theapplication of the energy, as, simultaneously, quasi-particles are beingcreated in the film II. The starting frequency of the phonon sweep isagain determined by the product of the duration and the power of theenergy pulse and the ending frequency of the phonon sweep is againdetermined by the the precision of the period between the energy pulses.This embodiment provides perhaps the simplest way to vary the orderparameter, although the precision obtainable in the control of thefrequency-range sweep may be somewhat less than that of the embodimentof FIG. 1. This is so because of inductive effects in the thin film 11both as the current pulse is started and terminated.

It is also known that the order parameter of a superconductance can bevaried by a variable magnetic field. This fact is employed in themodified embodiment of FIG. 3 by disposing the superconducting film 11in a strip line 41. An R-F field filed is propagated through the stripline 41 from a pulsed RF source 42. The field from source 42 ispropagated with its E field perpendicular to the metallic strips 43 and44 of strip line 41 and with its magnetic field parallel thereto. Thefilm 11 is disposed in a region where the magnetic field is maximumduring the propagation of the pulse. The space between strip conductors43 and 44 is typically filled by a low-loss dielectric 45 suitable foruse at liquid helium temperature.

A small hole is drilled through the magnetic strip 44 to accommodate thesample crystal 52 which is being investigated, and the detector diode 13is disposed on the opposite surface of sample 52. The R-F field pulse isprevented from being reflected in strip line 41 by an appropriateabsorbing termination 46.

In other respects the operation of the apparatus of FIG. 3 isessentially similar to that of the apparatus of FIG. 1.

FIG. 4 shows a modification of the embodiment of FIG. 3, which isstructurally more cumbersome because it employs an electromagnet 61.Nevertheless, this embodiment is conceptually nearly identical to thatof FIG. 3.

The magnetic field supplied by electromagnet 61 is aligned along theplane of the superconductive film I1 and is pulsed by a pulse of currentthrough the winding 62 from a pulsed current source 63. Theelectromagnet 6| has its induced north and south poles disposed onopposite sides of film 11 along the desired field direction. The controlof the frequencyswept phonons spectron may be achieved by adjustmentsofthe current pulse similar to those used in the embodiment of FIG. 2.ln other respects the operation of the embodiment of FIG. 4 is like thatofthe embodiment of FIG. 3.

Curve 91 of FIG. 5 illustrates a typical display that might be seen uponthe cathodelike tube-display device of any ofthe preceding embodimentsThe variation of a chirp" of phonons of rising frequency starts as theenergy pulse is turned off at frequency or time 0. The phonon frequencyincreases through values indicated on the abscissa of the graph of FIG.5. The detector signal passed through the superconducting detector diode13 varies as the absorption of the generated phonons in the sample 12 or52 varies. It is expected that at a particular frequency of thegenerated phonons a large absorption, which we term a resonantabsorption, may occur in the sample. This is shown by the sharp dip incurve 91. The phonon chirp is then illustratively completed without anysubstantial absorptions occurring at the higher frequency.

It should be apparent that the detector diode 13 in all cases is biasedto have an energy gap corresponding to the lowest frequency of thefrequency-swept phonon generator, so that all of the phonons may bedetected as the frequency of the generator varies. Nevertheless, all ofthe foregoing embodiments could be modified in the respect that thedetector diode energy gap could be swept in synchronism in thesuperconducting energy gap of the thin film 11. This could be done byvarying the order parameter of the superconductive films in diode 13according to the teaching of any of the embodiments of the presentinvention, so that the energy gap of the superconductor in the detectordiode l3 varies together with energy gap ofthe film 11, but alwaysremains slightly smaller.

What we claim is:

1. Apparatus of the type intended for use at superconductingtemperatures and comprising a superconducting device capable ofgeneration of phonons via recombination of quasiparticles,

means forexciting said quasi-particles in said device, and

means comprising material contacting said device for coupling saidphonons from said device for utilization, said apparatus beingcharacterized in that said device comprises a film of superconductingmaterial and characterized in the exciting means comprises an energysuperconducting to said film to supply energy capable ofsimultaneouslyvarying the superconducting energy gap of said film while excitingquasi-particles.

2. Apparatus according to claim 1 in which the energy source is a sourceof a pulse of light providing a total effect less than required toproduce the normal state in the film.

3. Apparatus according to claim I in which the energy source is a sourceofa pulse ofdirect current providing a total effect less than requiredto produce the normal state in the film.

4. Apparatus according to claim 1 in which the energy source is a sourceofa pulse ofmagnetic field providing a total effect less than requiredto produce the normal state in the film.

5. Apparatus according to claim I in which the coupling means includes asample of material to be investigated and includes utilization apparatuscomprising means for recording the time variation of phonons passedthrough said sample.

6. Apparatus of the type intended for use at superconduct ingtemperatures and comprising a superconducting device capable ofgeneration of phonons via recombination ofquasiparticles,

means for exciting said quasi-particles in said device, and

means comprising material contacting said device for coupling saidphonons from said device for utilization,

said apparatus being characterized in that said device comprises a filmof superconducting material of thickness less than approximately half ofthe London penetration depth and the exciting means comprises an energysource coupled to said film to supply energy capable of simultaneouslyvarying the superconducting energy gap of said film while excitingquasi-particles, whereby the phonons exhibit a predominant frequencyvarying in direct relation to said energy gap.

7. Apparatus according to claim 6 in which the device includes a stripline in which the film is disposed and the energy source includes apulsed microwave energy source coupled to said strip line.

8. Apparatus according to claim 6 in which the energy source is a sourceof an energy pulse providing a total effect less than that required toproduce the normal state in the film.

1. Apparatus of the type intended for use at superconductingtemperatures and comprising a superconducting device capaBle ofgeneration of phonons via recombination of quasi-particles, means forexciting said quasi-particles in said device, and means comprisingmaterial contacting said device for coupling said phonons from saiddevice for utilization, said apparatus being characterized in that saiddevice comprises a film of superconducting material and characterized inthe exciting means comprises an energy superconducting to said film tosupply energy capable of simultaneously varying the superconductingenergy gap of said film while exciting quasiparticles.
 2. Apparatusaccording to claim 1 in which the energy source is a source of a pulseof light providing a total effect less than required to produce thenormal state in the film.
 3. Apparatus according to claim 1 in which theenergy source is a source of a pulse of direct current providing a totaleffect less than required to produce the normal state in the film. 4.Apparatus according to claim 1 in which the energy source is a source ofa pulse of magnetic field providing a total effect less than required toproduce the normal state in the film.
 5. Apparatus according to claim 1in which the coupling means includes a sample of material to beinvestigated and includes utilization apparatus comprising means forrecording the time variation of phonons passed through said sample. 6.Apparatus of the type intended for use at superconducting temperaturesand comprising a superconducting device capable of generation of phononsvia recombination of quasi-particles, means for exciting saidquasi-particles in said device, and means comprising material contactingsaid device for coupling said phonons from said device for utilization,said apparatus being characterized in that said device comprises a filmof superconducting material of thickness less than approximately half ofthe London penetration depth and the exciting means comprises an energysource coupled to said film to supply energy capable of simultaneouslyvarying the superconducting energy gap of said film while excitingquasi-particles, whereby the phonons exhibit a predominant frequencyvarying in direct relation to said energy gap.
 7. Apparatus according toclaim 6 in which the device includes a strip line in which the film isdisposed and the energy source includes a pulsed microwave energy sourcecoupled to said strip line.
 8. Apparatus according to claim 6 in whichthe energy source is a source of an energy pulse providing a totaleffect less than that required to produce the normal state in the film.